In the early work within the field of the invention a large part of published work concerns microstructures made in silicon materials. Microfabricated structures made in silicon materials are often relatively easy to process further without harmful effects. In later work the material has changed to plastics.
A number of potential alternatives for adhering a lid to surfaces carrying open microchannel structures is at hand. Illustrative examples are by welding or by the use of various types of glues such as normal glues, hardening glues, thermal glues (including hot-melt adhesives) etc, each of which having their own benefits and pitfalls (WO 9845693, Aclara Biosciences; WO 9956954, Caliper Technologies). However, since the structures concerned are in the microformat and plastics surfaces often are heat-sensitive the risk for harmful changes in preformed microstructures have become significant (WO 996954, Caliper Technologies).
WO 9116966 (Amersham Pharmacia Biotech AB) discloses that sealing is accomplished by using a substrate material that is elastic, possibly in combination with gluing. Later it was suggested to use hot-melt gluing (WO 9429400, Amersham Pharmacia Biotech AB; and U.S. Pat. No. 4,957,582 Eastman Kodak). Thermolaminating a lid has been disclosed for microchannel structures that are to be used for electrophoresis (Soane et al., U.S. Pat. No. 5,858,188 and U.S. Pat. No. 6,054,034, Soane et al.; and McCormick et al., Anal. Chem. 69 (1997) 2626-2630).
These publications are directed towards microchannel structures in which the surface area in each individual structure is essentially homogeneous with respect to surface properties. Many times the structure has in principle only been intended to have one function (mass transportation) and the surfaces have had the initial hydrophobic/hydrophilic balance of the material used.
During the last years, the interest has focused on more complex structures in which there have been significant differences in surface characteristics between different parts of individual microchannel structures (so-called patterned microstructure surfaces). See for instance WO 9955827, WO 9958245, SE 9901100-9, and SE 9904802-7. It has become important to have simple, reproducible and reliable production methods for the manufacture of this kind of microfluidic devices.
The present invention is focusing on covering preprepared sets of open microchannel structures having parts that differ in surface characteristics. By the use of the invention the need for postdevelopment of surface characteristics in covered microchannel structures can be minimized.
Difficulties Encountered when Going from the Macroformat to the Microformat.
The nature of the difficulties often changes drastically to the worse, if miniaturising a macroscale system to a microscale system. Systems become more sensitive for evaporation and leakage, for instance. The demand on intra- and interset reproducibility in the manufacturing becomes more stringent. An acceptable deviation in the macroformat may become a functional disaster in the microformat. An acceptable physical deformation or a slight deviation from the intended hydrophobicity/hydrophilicity balance in a macrosystem, for instance a valve, a liquid conduit or a reaction and/or a detection zone, may in the corresponding microsystem render it impossible to accomplish reliable analysis results.
Thermal treatment of open microchannel structures made in plastics means a considerable risk for physical deformation and undesired changes of surface characteristics. In particular, hydrophilic surfaces on hydrophobic plastic substrates (almost all practically useful plastics are inherently hydrophobic) are very sensitive to heating (F Garbassi et al: Polymer Surfaces From Physics to Technology, p 324-337, Wiley 1998). The main reason is that the mobility of the polymer chain segments increases and allows for reorganisation of the surface so that polar hydrophilic groups turn inwards and nonpolar hydrophobic groups turn outwards.
Further the covered microchannel structures should resist pressure differences due to heating liquid-filled reaction chambers and the like. In microsystems the the mechanical stress levels involved may be greater than in macrosystems.